1. Field of Invention
This invention relates generally to a process for forming plastics. More specifically, this invention relates to a process, which utilizes conductive heating, for press-forming pre-laminated plastics from pre-cut plastic patterns.
Generally, in order to successfully mold a rigid piece of plastic into a different shape, the rigid plastic piece must first be heated. At a certain temperature, the rigid plastic piece becomes flexible. Once the plastic piece is sufficiently flexible, it is molded into the desired shape. After the plastic piece possesses the desired shape, the plastic piece is cooled and is thereby stabilized into the shape desired for the end product.
A few prior art plastic forming methods heat the plastic piece after the piece has been cut from a plastic sheet. On the other hand, most methods heat a specific section of a plastic sheet while that section is still part of the sheet, mold that section of the plastic sheet into the end-shape, and thereafter cut the plastic piece from the relevant section of the plastic sheet. Normally, the latter method stretches the relevant heated section of plastic sheet into the end-shape of the plastic piece thereby thinning the width of the resultant end product. Thus, it is preferable to use the former method, in which the end product is not made up of "thinned" plastic.
Generally, there are two types of heating which are used to heat plastic: [1] conductive or "contact" heating and [2] convective or "radiant" heating. Conductive heating occurs when the plastic is in direct contact with the heating element, wherein heat is transferred directly from the heating element to the plastic. Radiant heating occurs when the plastic is not in direct contact with the heating element, wherein heat is transferred indirectly from the heating element through the intermediary fluid (usually air) and to the plastic.
Each form of heating possesses drawbacks. For instance, in conductive heating, if one side of the plastic is exposed (ie., not heated), that side will likely "wrinkle" thereby making that piece of plastic unusable as an end product. Furthermore, using conductive heating, care must be taken so that the plastic does not melt into or adhere to the heating element. With respect to convective heating, since fluids tend to have lower thermal conductivities than solids, the degree of heat which the heating element must emit in radiant heating in order to raise the temperature of the plastic to an adequate level is usually much higher than the degree of heat required for conductive heating to raise the temperature of the same plastic to the same level. It would thus be beneficial to the prior art to develop an improved method of forming plastics which utilizes conductive heating but which does not suffer from the listed drawbacks of conductive heating.
The problems of convective heating are exacerbated if the plastic being heated is pre-laminated since the lamination and the plastic itself have different thermal conductivity coefficients. In addition, pre-laminated plastic sheets which are subjected to radiant heating typically do not heat evenly throughout, leading to difficulties in the molding step of the process. Nevertheless, the use of pre-laminated sheets is favored due to the additional long-lasting qualities which the lamination provides to the end product. Additionally, pre-laminated sheets are often desired for aesthetic reasons.
Two types of molding methods are predominantly used in prior art systems. Press molding compresses the plastic piece into its end shape, usually by using mating female and male dies. Vacuum molding utilizes a suction pressure to mold the flexible, heated plastic piece into its end shape. An inherent limitation in vacuum molding is that its efficiency and precision depends largely on the power of the vacuum/suction mechanism. It would thus be beneficial to the prior art to develop an improved method of forming plastics which utilizes press-molding instead of vacuum molding.
The great majority of prior art systems utilize water cooling mechanisms to cool the plastic subsequent to its heating phase. Other systems utilize air or a combination of water and air to achieve the same purpose. Using water based (including partially water based) cooling methods requires complicated equipment for routing and pumping the water towards the heated plastic. It would thus be beneficial to the prior art to develop an improved method of forming plastics which utilizes air cooling.
Collectively, and more particularly, it would be beneficial to develop a method of forming plastics which heats a plastic piece after it has been cut from a sheet of plastic (thereby eliminating the "thinning" plastic problem), utilizes conductive heating to heat the plastic piece on both of its sides while providing a means to inhibit melting of the plastic, utilizes press-molding instead of the more limiting vacuum molding, and utilizes air cooling to cool the heated and molded plastic piece (thereby eliminating the need for a complex water cooling mechanism). Further, it would be beneficial to the prior art to provide a method which can effectively form pre-laminated plastic sheets.
2. Related Art
Although methods of forming plastics are known in the art, the Applicant is unaware of a prior art method which utilizes the steps as previously listed herein. Specifically, the Applicant is unaware of a prior art method which utilizes the steps as previously listed herein to form a pre-laminated plastic sheet. Illustrative of known prior art methods are U.S. Pat. No. 2,305,433 which issued to Kyle in 1942, U.S. Pat. No. 3,020,596 which issued to Clapp et al. in 1962, U.S. Pat. No. 3,084,389 which issued to Doyle in 1963, U.S. Pat. No. 3,404,056 which issued to Baldwin in 1968, and U.S. Pat. No. 4,275,864 which issued to Richards in 1981.
U.S. Pat. No. 2,305,433 ("the '433 Patent") discloses a Method and Apparatus for Molding Articles of Thermoplastic, Thermosetting, or Resinous Materials. The '433 Patent utilizes two sets of male and female dies (one set for heating and one set for cooling) and at least one shell, which assembly allows for the quick re-use of the set of heating dies.
Although the '433 Patent discloses heating both sides of a plastic piece by use of pressure and contact, the type of heating taught by the '433 Patent is not pure and direct contact heating. In the '433 Patent, heat is transferred from one or two heating dies through an adjacent shell, which is not part of the end-product and is subsequently removed, and finally into the plastic. The indirect conductive heat transfer taught by this Patent results in a lower overall thermal conductivity coefficient than if direct contact heating were used. It is noted, however, that the '433 Patent would not operate without the use of the disclosed shells.
Furthermore, the '433 Patent teaches that the heating and molding steps of the method occur concurrently. The shape of the end product for this method is thus inherently limited to the shape of the mold/heating structure. On the other hand, if the heating and molding steps are carried out independently from each other, the plastic piece, once it has been heated, may be used in a vast number and shape of molds.
U.S. Pat. No. 3,084,389 ("the '389 Patent") discloses a Plastic Molding Process and Apparatus. The '389 Patent teaches a faster cooling method for a heated piece of plastic, comprising the sequential steps of skin hardening the plastic with a cooling gas (ie., air) and then spraying the plastic with a water or liquid mist. The '389 Patent does not utilize contact heating and utilizes vacuum molding instead of press molding.